Drug discovery and development consists of an arduous testing process, beginning with the demonstration of pharmacological effects in experimental cell and animal models and ending with drug safety and efficacy studies in patients. It is estimated that only 1 out of 5,000 screened compounds receives FDA approval as a safe and effective new medicine. Approximately 25% of compounds are eliminated in pre-clinical toxicological studies. Thus, a significant number of drug candidates in pre-clinical development fail to progress out of this stage due to unacceptable levels of toxicity in test systems.
Multiple pharmacologic parameters are considered when evaluating a drug candidate. Knowledge of the absorption, distribution, metabolism and excretion profile (“ADME”) of a drug and its metabolites in humans (and animals used in toxicology assessments) is crucial to understanding differences in effect among individuals in a population and for optimizing dosimetry. Absorption and bioavailability are standard measures of the amount of biologically active material distributed to the systemic circulation or local site of action. Duration of drug action is often dependent on how rapidly the body eliminates the active molecules, either through metabolism, which involves chemical modification by drug-metabolizing enzymes, or by excretion, which involves binding and transport away from biologically active sites in the body. Thus, typical pre-clinical studies involve monitoring permeation across epithelial membranes (e.g., gastrointestinal mucosa), studies of drug metabolism, identification of plasma protein binding and evaluation of transport into and out of tissues, especially organs that eliminate drug products, such as the kidney and liver.
New Drug Applications, or NDAs, are submitted on the basis of data obtained from a small number of patients (>10,000), which is usually not indicative of the general population at large. Often, limited toxicity is observed and the selected patients have relatively normal organ function. Toxicity refers to any unwanted effect on normal structural or functional integrity. A toxic dose refers to a dose producing an unwanted or overly exaggerated pharmacological effect in a subject (i.e., the dose received by a subject when the first truly toxic signs develop). Failure in development of candidate drugs often occurs when an unacceptable level of toxicity develops.
Often, healthy subjects take part in early stage clinical studies. However, patient responses to drugs are typically more complex and less predictable than responses in the healthy subjects. The chances of adverse drug effects in patients are greatly increased due to increased susceptibility (e.g., increased susceptibility due to drug to drug interactions and comorbid conditions). There can be significant differences in toxicology and metabolism among groups of patients that are not detected until the after drug has advanced from pre-clinical studies. In the worst case, significant effects on a patient population are not detected until after a drug receives approval from the Food and Drug Administration. Therefore, pre-clinical approaches to identify toxicity in the early phases of drug discovery represent an important step towards efficient drug development.
Current pre-clinical toxicity and pharmacology studies utilize in vitro assays involving cultured cells or subcellular organelles, as well as in vivo animal models to investigate drug metabolism, toxicity and possible efficacy. While technological advances in cell, molecular, and biochemical assays have made significant strides, a number of problems still exist. First, in vitro assays using purified or recombinant enzymes and cell cultures provide the first step in determining pharmacologic and toxicologic parameters to be used thereafter in animal models, but are often too simplistic to account for the multifactorial events that occur during drug metabolism in a human system or human organ. Second, data obtained in animal models cannot be reliably extrapolated to human systems. Third, many drugs used to treat chronic diseases such as HIV infection or Alzheimer's disease necessitate dosing regimens that are applied over long periods of time, and in some cases, over the lifetime of an individual. Currently, development of chronic toxicity is most practically observed during long-term patient use.
The high attrition rate of drug candidates is a major economic deterrent in the pharmaceutical industry, as drug failure may be identified only after great time and expense are invested. These failures can be attributed, in part, to a lack of effective pre-clinical models and assay systems. Accordingly, there is a great need in the art to develop an in vitro human system that can effectively evaluate the pharmacologic and toxicologic properties of drug candidates. Improved in vitro model systems will allow the drug development process to reliably predict the in vivo response before the drug reaches the clinic, decreasing time, expense and significant risks to patient health. In addition, many serious diseases (e.g., hepatitis C) lack reliable animal model systems, a problem that has severely handicapped the drug discovery process. Improved in vitro model systems will also provide an opportunity for meaningful pre-clinical experimentation, which is essential for the development of therapeutics in the absence of animal model systems.